Radiation detecting device



y 1953 s. A. SCHERBATSKOY RADIATION DETECTING DEVICE Filed Dec. 15, 1950 INDICATOR F IG. 2

VOLTAGE FIG. I

Patented May 19, 1953 UNITED s'rArEs PATENT, OFFICE RADIATION DETECTING DEVICE Serge A. Scherbatskoy, Tulsa, Okla.

Application December 15, 1950, Serial No. 200,920

I 4 Claims. (01. 250-8341) My invention relates to radiation detecting devices and more particularly to a device of the counter type for detecting and measuring penetrative radiation such as gamma rays, neutrons, beta rays, and other atomic and nuclear particles.

The radiation counter operates by the production of an electrical discharge in a gas. The counter is extremely sensitive and a discharge may be produced when a single ion pair is produced almost anywhere in the tube. It is extremely diflicult to assure that a counter even very carefully made to any specification will be satisfactory and uniform in all its characteristics. The extreme sensitivity of the counter renders it liable to drifts and spontaneous discharges, the origin of which is uncertain. Furthermore, the characteristics of the discharge are also often found to change with time, temperature, and exposure to radiation, and other factors.

It is the purpose of my invention to compensate for drifts in a radiation counter and to provide an automatic controlling arrangement that will insure the stability of operation of the device.

It is another purpose of my invention to compensate for changes in counter characteristics and to provide a control arrangement for automatically adjusting the voltage supply in response to any departure of the counter from normal operating condition.

Other objects, together with some of the advantages to be derived in utilizing the present invention, will become apparent from the following detailed description, taken together with the accompanying drawings, wherein:

Fig. 1 shows' a typical characteristic curve of a radiation counter.

Fig. 2 shows diagrammatically an automatic circuit for varying the voltage supply of radia' tion counter in response to the departure of the counter from normal operating conditions.

Referring now more particularly to Fig. 1 there is shown a diagramillustrating the characteristic of a radiation counter. The diagram is in form of a graph showing the counting rate against voltage applied between cathode and anode for a constant intensity of irradiation, as for example, with the gamma rays from one milligram of radium at a determined distance from the counter. Theshape of the curve depends partly on the circuit. E5 is by definition the starting potential, i. e. the voltage which must be applied to a counter to cause it to count and below which no discharges occur. E6 is the threshold potential; it corresponds to the beginning of the strictly nonproportional region. After reaching the voltage ED the counting rate begins to rise again due to self-excitation and production of spurious counts. Region P, comprised between the voltages EG and ED is known as the plateau.

The plateau is the region. at which satisfactory operation of the counter occurs.

. than 1000 volts.

Let Es designate the operating voltage, 1. e. the voltage at which the counter is operated. This is the voltage across the counter measured between the cathode and anode. As shown in Fig. l', the plateau is of limited width and, consequently, a radiation counter operates satisfactorily over a relatively limited range of voltages. For example, some counters operate satisfactorily at Eo 1050 volts but willcease operating properly at a voltage much higher than 1100 volts and will also cease operating at a voltage much lower Many Geiger counters have a plateau which is sometimes no larger than a few per cent of the operating voltage.

It is also well known that the operating voltage of a Geiger counter is not always exactly fixed. Also, it is very difiicult to manufacture Geiger counters all having exactly the same operating voltage. The usual technique of operating Geiger counters involves the following steps:

. (a) Determine the starting voltage of the Geiger counter.

y (b) Determine the length of the plateau and the voltage at which spurious discharges begin to occur, or the voltage at which excessive curvature of the plateau occurs.

(0) Select the operating voltage. at a suitable point in the plateau region. There are a num-,

ber of accepted procedures for the selection of the operating voltage. cedure, the operating voltage should exceed the starting voltage by a certain fixed amount E1, i. e.

Eo Es+EI the counter.

It is apparent that when the counter is operatedin the Geiger region as a self-quenching counter, each discharge caused by the passage of a radiation particle is accompanied by a voltage impulse, i. e. the voltage across the counter measuredbetween the cathode and anode is suddenly reduced from the initial value represented by. the operating voltage E0 toa final value closerto the starting voltage Es at which the counter .becomes extinguished. There usually is a definite relationship between the magnitude of the voltage impulse Ekand the difference between E0 and Es. impulse E1; has the approximate magnitude According to one pro- In some counters the voltage condenser 26 a volta ge w 1s 1 As stated above, the characteristics of the counter are usually notstable and due to conditions beyo'ridour control'the starting yoltage of the counter may undergo unpredictable changes. It would be then desirable to vary the -5 operating voltage E in order to follow these changes so as to maintain the valiie'E'iZ My invention provides an arrangement in which these variations of the operating voltage E; are effected automatically. t. t

It is apparent from Formula 1 that when the starting voltage E5 unexpectedly increases (or conversel decreases) and the operating voltage Ea remains the same; their the; tort-age impulses Eit'correspondingly decrease (or increase); accordance with my invention 1." utilize the d" crease (or increase) of the impulses aslacan; trolling factor in order to adj'iistlutoiiiatically a suitable controlling device". tunnell ng device increases'ior conversely decreases) the 0113-; crating voltage E5 order t6 restore the impulses El';.t0 their previous values H .UfIhus the 'cbntrollin'g device adjusts the 61 crating voltage in' response to the output irijpnlses in order to restore these impiilses to their nbrih'al operating magnitude.

An instrumental arrangement :i'or perfecting such an automatic control is Shawn schematically inFig; 2. The conventional radiation counter 9 shown in Fig. 2 consists of a thin walled metal tube In with a very thin wire I l fdrfrii'ng the anode spanned axially and insulated from theaters-1 tube forming the cathode; 'r ieseeiectrgcee are enclosed in an envelope l2 cornmei'ily glass tube which contains a suitable gas in F mainly argon, at a fairly lbw e es 7 v cni. of Hg. The centrerwire n is maintained at a positive potential-With respect retire cylinder It) by means of a high voltage supply IQ (which can beta battery) in series with fairly highresistors I4, l5 placed in the circiiit, If a particle capable of ionizing the gas passes throug the cylinder in; a discharge will take place and a current pulse flows through the resistor 14. causes a voltage drop across the resistor I4 and the discharge will ceaseafter a very short period of time. Suitable treatment of the surface of h cylinder s an pr er. e qfi o the. a e filling the counter will eagise the discharge to Step more ul iztlre g e Each dischargeof the tj ciii.ri ter p; a e imtuise betw mfie am e These voltage mpulses; are tr a coul ling condenser l1 Another resistor 21 is co anode 2B ofthe rectifier 2] a arrangement jee' rising re voltmeter and it i ttc pted t q i to the e v ue ime Q11 ee d l across the amplifier 'l 8 The cathodefn di the rectifier is c nnecte the g d 3' t. The ube 31 has ts 15; "t' v juncti n t i el 1 the eisst s 1' cath0i1e'33 of the tube 3| is cdn'nected to the ground by means ofa resistance condenser ele- 4 ment, said element consisting of a resistance 36 shunted by -the c ndenser 3L i .etrtie j h errit 31 j fisti a shunt applied to the resistor 15 in series With the high voltage supply l3 and that the equivalent impedance of the shunt may be appropriate- Iy' varied by means of the voltage applied to the lq n r X s Th tiibe 31 is provided with a screen grid 39 whichisconnected to the ground by means of resistor fifi, the other terminal of said resistor being connected through resistors 4|, 42 to the cathode 33. The tube 3| is energized by a b attery 43' having its negative terminal grounded and the positive termin'aiecnnected to the'junc' tion or resistors 4i and 42; I N

It is ap arent that normally in the absenceci any radiation the grid 3!! mamt ined at Zerti otential arid the "cathode at a positive potential derired from the b'ettt ry 43; v

The dfierz'ttlofi 0f the above arrangement is as follows: u .t p a I v The Gige'ricounters is energized by thel iigh voltage supply ,l3'. This high voltage supply is chosen to havefa substantialry higher volta e than will be requiredfdr tl'ie'operatien bf the Geiger counter. Under nonfien operating ou ditions, even iii the or my ex rnai radia tion, there is a small 'ciir'reiit fiovi/i g frorri the battery 13 through a circu t consisting or the resistor [5 in series with the variable resistance tube 3i tea the resistance capacitance element 36-31; This current causes a voitege drop across the resistor l5. The actual ep'eretm'g voltage Ed applied tcthe ce'unter ccnsiste er the an;

ference of potential between the terminals 5 l and" 52 and is equal to the voltage of the battery I 3 decreased by the potential crap across the i'esis tor .l5';

Itis apparentthat by varying the potential applied to the grid 30 of the tube 3! the equivalent resistance of the tueevanes. This in turn causes variation in the clirreiit throughtlie resistor l5 and consequently the variation of the operating voltage E8 derived f'rb'm the terminals 5|; 52; censequent by a'ryingtn-e grid voltage of thetube 3| we effect a corresponding varia tion in the eperetmg vbltage 6f the counter.

MUncler normal operating cancerous whenever the counter discharges due 'td a passage off a radiation particle, we obtain a sudden voltage impulse at the terminal 53 of the anode. These voltage impulses are applied in a conventional manner: to the amplifier l8 and subsequently rectified by means of the rectifier 2!. The action of the rectifier and condenser 26 and resistor 25 is such that e D. C. voltage develops across condenser 26. which is substantially equal to the peak v'alue'bf the voltage d tect or the amplifier ofjthe circuit so that the time 'o'onst'ant deter mined by condenser 26; internal resistance of rectifier 2|; and the output resistance of emthus mainta ns the acetates v itt'e'e tr the tube sew that I a a I ditions beyond our control, the starting voltage Es drifted and assumed a lower (or, conversely a higher) value Es. Assume also that the operating voltage is maintained at its value E0. Consequently, the voltage impulses Er, applied to the terminal 53 increase (or conversely decrease). in magnitude and assume a new value E'Ic:Eo-E's. Consequently, the rectified current passing through the resistor 25 increases (or conversely decreases) in intensity and causes corresponding increase (or decrease) of the voltage at the condenser 26. This increase in voltage applied to the grid 39 of the tube 3| causes a corresponding decrease (or, conversely, increase) in the equivalent resistance of the tube. Consequently, the corresponding current delivered by the battery 13 and passing through the resistor l increases (or, conversely, decreases) in magnitude. This in turn increases (or, conversely, decreases) the voltage drop across the resistor 15. Therefore, we obtain at the terminals 5|, 52 a new value of the operating voltage E's that has changed in magnitude so as to countereffect the drift in the starting potential. The new value E'o is such that Ek=E's-E'0 i. e. the voltage impulses E1; are automatically maintained at a constant value.

It is thus apparent that if the voltage impulses developed by the counter are too large (or, conversely, too small) the circuit automatically readjusts itself and decreases (or, conversely, increases) the voltage supplied until it has a lower (or, conversely, higher) value and countereffects the increase (or, conversely, decrease) of the voltage impulses supplied by the counter.

The controlling circuit used in the above arrangement is shown in Fig. 2 enclosed within a dotted line 50.

It is thus apparent that I have provided a radiation counter with a controllable voltage supply for applying a controllable voltage between the electrodes H and I2 of the counter. The controllable voltage supply consists of the high voltage supply I3, in series with the resistor 15 and the variable shunt element comprising the tube 3|. Under normal operating conditions the counter produces impulses of a determined magnitude Ek. It may happen, however, that because of some efiects beyond our control the impulses El: have changed their magnitude and assumed a new value E'k. It is then desirable to have an automatic circuit that will restore the counter to normal operating conditions and will restore the impulses to their normal value E11. I accordingly provide such an automatic circuit that is arranged to be responsive to any departure of the impulses from the normal value Es, and is effective to modify the variable shunt eleoperative to produce voltage pulses, the intensity of said radiation being indicated by the frequency of said pulses, in combination with a D.-C. voltage source for said discharge chamber and control means for said voltage source coupled to said discharge chamber operative to vary the D.-C. voltage output of said voltage source responsively to changes in the magnitude of said pulses.

2. A radiation counter of the type comprising a radiation-sensitive element, of the: type wherein radiation traversing said element is operative to produce voltage pulses, the intensity of said radiation being indicated by the frequency of said pulses, in combination with an amplifier coupled to said radiation-sensitive element for amplifying said pulses, a D.-C. voltage source for said element, and control means for said voltage source coupled to said amplifier operative to vary the voltage output of said voltage source responsively to changes in the magnitude of said pulses.

3. A radiation counter of the type comprising a gas-filled discharge chamber, an impedance element, and a voltage supply in series wherein radiation traversing said discharge chamber is operative to produce voltage pulses across said impedance element, the frequency of said pulses representing the intensity of saidradiation, in combination with an amplifier coupled to said impedance element for amplifyingsaid pulses,

indicator means fed by said amplifier, and stabilizing apparatus comprising rectifier means having an input and an output, said input being coupled to said amplifier and said output being connected in shunt with an electric network comprising a resistor and a capacitor in parallel, and control means for said voltage supply operative responsively to changes in the potential across said electric network to change the voltage output of said voltage supply.

4. Radiation-measuring apparatus comprising in combination a gas-filled discharge chamber, a D.-C. voltage supply, two impedance elements, said impedance elements being connected to form a series loop with said voltage supply and said discharge chamber, said discharge chamber being operative to conduct current transiently in response to radiation traversing said chamber and thereby to produce voltage pulses at a rate governed by the intensity of such radiation, an electron-discharge device comprising a spaceconduction path and a control element therefor, said space-conduction path being connected in parallel with the series circuit comprising one ment and thus to effect an appropriate control upon the voltage supplied to the counter. As a result of such a control the voltage between the electrodes of the counter has been changed in such a manner as to restore the magnitude of the impulses to their normal operating value Ek.

While I have described my invention in connection with certain specific embodiments thereof, it is to be understood that this is by way of illustration and not by way of limitation and I do not mean to be bound thereby, but only to the scope of the appended claims which should be construed as broadly as the prior art will permit.

I claim:

1. A radiation counter of the type comprising a gas-filled discharge chamber, of the type wherein radiation traversing said discharge chamber is of said impedance elements and said discharge chamber, an amplifier coupled to said discharge chamber for amplifying said pulses, indicator" means fed by said amplifier, and means coupled to the output of said amplifier and to said control element operative responsively to changes in the 'pulse magnitude to vary the potential. on said control element.

SERGE A. SCHERBATSKOY.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,222,451 Trost Nov. 19, 1940 2,407,853 Smith Sept. 17, 1946 2,443,006 Johnson June 8, 1948 2,465,938 Shonka Mar. 29, 1949 2,475,613 Hastings July 12, 1949 2,477,802 Herzog et al Aug. 2, 1949 2,496,886 Molloy et a1. Feb. 7, 1950 

